Flipped gate current reference and method of using

ABSTRACT

A current reference comprising: a control resistor; a tracking voltage generator including: a first, flipped gate transistor having a first size; and a second transistor having a second size; wherein the tracking voltage generator is configured to output a tracking voltage having a first temperature dependency based on a ratio of the second size to the first size, the temperature dependency thereby being substantially equal to a second temperature dependency of the control resistor; and an amplifier circuit configured to receive the tracking voltage and maintain a voltage at a first terminal of the control resistor, the voltage being substantially equal to the tracking voltage; wherein the current reference thereby is configured to maintain a reference current through the control resistor at a constant value.

PRIORITY CLAIM

The present application is a continuation of U.S. application Ser. No.14/518,638, filed Oct. 20, 2014, forthcoming as U.S. Pat. No. 9,590,504,which claims priority of U.S. Provisional Application No. 62/057,567,filed Sep. 30, 2014, the disclosures of which are incorporated herein byreference in their entireties.

RELATED APPLICATIONS

This application is related to U.S. application Ser. No. 14/182,810,filed Feb. 18, 2014, entitled FLIPPED GATE VOLTAGE REFERENCE AND METHODOF USING, published as U.S. Pre-Grant Publication No. US2015/0234413;and to U.S. application Ser. No. 14/451,920, filed Aug. 5, 2014,entitled FLIPPED GATE VOLTAGE REFERENCE HAVING BOXING REGION AND METHODOF USING, published as U.S. Pre-Grant Publication No. US2015/0234412,both of which are herein incorporated by reference in their entireties.

BACKGROUND

A current reference is a circuit used to provide a reference current toa circuit. The circuit uses the reference current as a means of biasingvarious components within the circuit. For example, current referencesare used to provide an accurate bias for components such as oscillators,amplifiers, phase locked loops, or other suitable components.

In some approaches, the current reference is formed using bipolarjunction transistors (BJTs) to form bandgap references to provide thereference voltage signal. In PNP BJTs the substrate acts as a collectorfor the BJT rendering the BJT sensitive to majority carrier noise in thesubstrate. In NPN BJTs the collector is formed as an n-well in a p-typesubstrate and is susceptible to picking up minority carrier noise fromthe substrate. Neither NPN BJTs or PNP BJTs allow full isolation fromsubstrate noise.

In some approaches, a temperature independent resistor, such as asilicon chromium resistor, is used in combination with a voltagereference. In some approaches, a proportional to absolute temperature(PTAT) current source is used in combination with a complementary toabsolute temperature (CTAT) current source to reduce temperaturedependency of a generated current.

BRIEF DESCRIPTION OF THE DRAWINGS

One or more embodiments are illustrated by way of example, and not bylimitation, in the figures of the accompanying drawings, whereinelements having the same reference numeral designations represent likeelements throughout. It is emphasized that, in accordance with standardpractice in the industry various features may not be drawn to scale andare used for illustration purposes only. In fact, the dimensions of thevarious features in the drawings may be arbitrarily increased or reducedfor clarity of discussion.

FIG. 1 is a schematic diagram of a current reference in accordance withsome embodiments.

FIG. 2 is a cross sectional view of a flipped gate transistor inaccordance with some embodiments.

FIG. 3 is a schematic diagram of a tracking voltage generator inaccordance with some embodiments.

FIG. 4 is a schematic diagram of an amplifier in accordance with someembodiments.

FIG. 5 is a schematic diagram of a current mirror in accordance withsome embodiments.

FIG. 6 is a schematic diagram of a current mirror in accordance withsome embodiments.

FIG. 7 is a top view of a resistor arrangement in accordance with someembodiments.

FIG. 8 is a flow chart of a method of using a current reference inaccordance with some embodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the invention. Specificexamples of components and arrangements are described below to simplifythe present disclosure. These are examples and are not intended to belimiting.

FIG. 1 is a schematic diagram of a current reference 100 in accordancewith some embodiments. Current reference 100 includes a tracking voltagegenerator 110 configured to generate a tracking voltage V_(TRK). Currentreference 100 further includes an amplifier 120 configured to receivethe tracking voltage V_(TRK) at a first input of the amplifier.Amplifier 120 is further configured to selectively adjust a conductivityof a control transistor M11. A feedback loop is connected to a secondinput of amplifier 120. An output of amplifier 120 is connected to agate of control transistor M11. A source of control transistor M11 isconnected to a first end of a control resistor R11 in series. Thefeedback loop is connected to the source of control transistor M11between the control transistor and control resistor R11. A second end ofcontrol resistor R11 is connected to a negative supply voltage VSS.Current reference 100 further includes a current mirror 130 connected toa drain of control transistor M11. Current mirror 130 is configured tomirror a current passing through control transistor M11 to externalcomponents.

Tracking voltage generator 110 is configured to generate a temperaturedependent tracking voltage V_(TRK). The temperature dependence oftracking voltage V_(TRK) is substantially equal to a temperaturedependence of a resistance of control resistor R11. As a resistance ofcontrol resistor R11 increases due to temperature variation, a voltagelevel of tracking voltage V_(TRK) increases at substantially a same ratein order to maintain a substantially constant ratio V_(TRK)/r11, wherer11 is the temperature dependent resistance of control resistor R11.Similarly, as the resistance of control resistor R11 decreases due totemperature variation, the voltage level of tracking voltage V_(TRK)decreases to maintain the ratio V_(TRK)/r11 at a substantially constantvalue.

Tracking voltage generator 110 includes a flipped gate transistor M1between an operating voltage VDD and negative supply voltage VSS. Afirst current source 112 is configured to supply a first current I1across flipped gate transistor M1. A transistor M2 is connected betweenoperating voltage VDD and negative supply voltage VSS. Transistor M2 isconnected to flipped gate transistor M1 in a Vgs subtractivearrangement. A Vgs subtractive arrangement means that an output oftracking voltage generator 110 is equal to a Vgs voltage of flipped gatetransistor M1 minus a Vgs voltage of transistor M2. The Vgs subtractivearrangement results from a gate of transistor M2 and flipped gatetransistor M1 receiving a same voltage and a source terminal of theflipped gate transistor M1 connected to negative supply voltage VSS. Asecond current source 114 is configured to supply a second current I2across transistor M2. A transistor M3 is connected between transistor M2and negative supply voltage VSS. Each of a gate, a source terminal, anda bulk of transistor M3 are connected to negative supply voltage VSS. Anoutput node for outputting tracking voltage V_(TRK) is located betweentransistor M2 and negative supply voltage VSS and is connected to adrain terminal of transistor M3.

Flipped gate transistor M1 is used to help produce a temperaturedependent tracking voltage V_(TRK). Flipped gate transistor M1 includesa gate electrode which is anti-doped. Anti-doping is a process of dopingthe gate electrode with a dopant type which is the same as a substrateof flipped gate transistor M1. For example, in a conventional n-typemetal oxide semiconductor (NMOS), the substrate is p-doped and the gateelectrode is n-doped. However, in a flipped gate NMOS; a portion of thegate electrode is p-doped.

FIG. 2 is a cross sectional view of a flipped gate transistor 200 inaccordance with one or more embodiments. Flipped gate transistor 200 isan n-type flipped gate transistor. Flipped gate transistor 200 includesa substrate 202. A gate dielectric layer 204 is over a channel region206 of substrate 202. A gate electrode 210 is over gate dielectric layer204. A body region 212 of gate electrode 210 is doped with p-typedopants. Edges 214 of gate electrode 210 are n-doped for self alignedformation of n-doped source/drain (S/D) features 220. Isolation regions230 are positioned between adjacent flipped gate transistors, in someembodiments. In some embodiments, gate electrode 210 includes dopedpolysilicon, a metal gate or another suitable gate material. In someembodiments, the p-type dopants include boron, boron di-fluoride, orother suitable p-type dopants. In some embodiments, the n-type dopantsinclude arsenic, phosphorous, or other suitable n-type dopants.

Returning to FIG. 1, the gate of flipped gate transistor M1 is connectedto a drain terminal of the flipped gate transistor. A bulk of flippedgate transistor M1 is connected to the source terminal of the flippedgate transistor. In some embodiments, flipped gate transistor M1 issubstantially p-doped. Substantially p-doped means that a gate electrodeof flipped gate transistor M1 is p-doped except at edges of the gateelectrode. The edges of the gate electrode of flipped gate transistor M1are n-typed to facilitate forming of the drain and source terminals ofthe flipped gate transistor.

First current source 112 is configured to supply the first current toflipped gate transistor M1. In some embodiments, first current source112 includes at least one current mirror. In some embodiments, firstcurrent source 112 includes a startup device and a current generationdevice, an external current source, or another suitable current source.

Transistor M2 is used to help produce the temperature dependent trackingvoltage V_(TRK). Transistor M2 is not a flipped gate transistor. In someembodiments, transistor M2 is a standard NMOS transistor. The gate oftransistor M2 is connected to the gate of flipped gate transistor M1. Adrain terminal of transistor M2 is connected to operating voltage VDD. Abulk of transistor M2 is connected to the source terminal of thetransistor.

Flipped gate transistor M1 has a first size defined by a width and alength of the flipped gate transistor. Transistor M2 has a second sizedefined by a width and a length of the transistor. The size oftransistor M2 is greater than a size of flipped gate transistor M1. Thesize of transistor M2 is an integer multiple N of the size of flippedgate transistor M1. In some embodiments, the integer multiple N rangesfrom about 2 to about 50. In some embodiments, the size of a transistoris defined based on a number of fingers with each finger having a samewidth W and a same length L. For example, a transistor having twofingers having width W and length L has a size two times greater than atransistor having one finger having width W and length L. A sizedifference between transistor M2 and flipped gate transistor M1 helpsdetermine a temperature dependence of tracking voltage V_(TRK). In someembodiments, the length of flipped gate transistor M1 and the length oftransistor M2 are substantially equal, so the size difference isdetermined based on a difference between the width of flipped gatetransistor M1 and the width of transistor M2. For example, in someembodiments, flipped gate transistor M1 has a width W1 and a length Lgiving the flipped gate transistor a size of W1/L; and transistor M2 hasa width W2 and a length L giving the transistor a size of W2/L, so aratio of the size of transistor M2 to the size of flipped gatetransistor M1 is given by W2/W1. In some embodiments, a ratio of W2/W1ranges from about 2 to about 50. In some embodiments, the sizedifference between transistor M2 and flipped gate transistor M1 isadjusted by altering a number of fingers in transistor M2 or flippedgate transistor M1. For example, in some embodiments, transistor M2 has2 times to 50 times more fingers than flipped gate transistor M1. When asize of flipped gate transistor M1 is equal to a size of transistor M2and a current across both flipped gate transistor M1 and transistor M2is equal tracking voltage V_(TRK) has a negative variation withtemperature, in some embodiments. Adjusting the relative size of flippedgate transistor M1 with respect to transistor M2 alters how trackingvoltage V_(TRK) varies with respect to temperature. Temperaturedependency of tracking voltage V_(TRK) is also dependent on a currentdensity of transistor M2 relative to a current density of flipped gatetransistor M1. Proper sizing of transistor M2 relative to flipped gatetransistor M1 or setting of a current density through transistor M2relative to flipped gate transistor M1 results in a temperaturedependency of tracking voltage V_(TRK) which substantially matches thetemperature dependency of control resistor R11.

First current source 112 is configured to provide the first current toflipped gate transistor M1. Second current source 114 is configured toprovide the second current to transistor M2. A least common denominatorcurrent (I_(LCD)) is defined based on a ratio of the first current tothe second current. For example, a ratio of the first current to thesecond current being 11:2 results in a least common denominator currentof 1. A ratio of the first current to the second current being 8:4results in a least common denominator current of 4. The first current isa first integer multiple (K1) of the I_(LCD). The second current is alsoa second integer multiple (K2) of the I_(LCD). The first integermultiple K1 is greater than the second integer multiple K2. In someembodiments, the first integer multiple K1 is about two times greaterthan the second integer multiple K2. In some embodiments, the firstinteger multiple K1 is more than two times greater than the secondinteger multiple K2.

The integer multiple N is determined at least in part by first integermultiple K1 and second integer multiple K2. Tuning of integer multiple Nenables adjustment of temperature dependency of tracking voltageV_(TRK). Tuning the integer multiple N, such that the ΔV_(gs) of flippedgate transistor M1 and transistor M2 is approximately equal to thebandgap voltage of a semiconductor-based material used in productionprocess used to form tracking voltage generator 110, results indecreased temperature dependency of tracking voltage V_(TRK). In someembodiments, where control resistor R11 has a temperature independentresistance, the ΔV_(gs) of flipped gate transistor M1 and transistor M2is set as approximately equal to the bandgap voltage of asemiconductor-based material to generate a temperature independenttracking voltage V_(TRK).

Transistor M3 is used to remove a channel leakage component of a drainsource current running through transistor M2. A size of transistor M3 isequal to a size of transistor M2. Any leakage current through transistorM2 is directed to transistor M3 to help maintain the second current I2for the purpose of temperature compensation of the tracking voltageV_(TRK). The addition of transistor M3 to compensate for leakage throughtransistor M2 helps to use an entirety of the second current I2 for thepurpose of temperature compensation for tracking voltage V_(TRK). Thisleakage cancellation is most effective when the drain-source voltage ofM2 is equal to the drain-source voltage of M3, which happens whenoperating voltage VDD is set at a value given by 2 V_(TRK). Inapproaches that do not include transistor M3, accuracy of the trackingvoltage generator rapidly degrades at temperatures above 80° C.

Amplifier 120 is configured to receive tracking voltage V_(TRK) at thefirst input and to receive a feedback voltage V_(FB) at the secondinput. The output of amplifier 120 is connected to the gate of controltransistor M11 to adjust conductivity of the control transistor.Amplifier 120 in combination with control transistor M11 is configuredto maintain tracking voltage V_(TRK) substantially equal to feedbackvoltage V_(FB). By maintaining tracking voltage V_(TRK) substantiallyequal to feedback voltage V_(FB), a reference current Iref acrosscontrol resistor R11 remains constant due to the constant ratio ofV_(TRK)/r11 for a wide range of temperatures.

The source of control transistor M11 is connected in series with controlresistor R11. A drain of control transistor M11 is connected to currentmirror 130. In some embodiments, control transistor M11 is an NMOStransistor.

Control resistor R11 is connected between control transistor M11 andnegative supply voltage VSS. In some embodiments, control resistor R11has a temperature independent resistance. In some embodiments where theresistance of control resistor R11 is temperature independent, thecontrol resistor includes the use of silicon chromium (SiCr) resistors.

In some embodiments, control resistor R11 has a negative temperaturecoefficient (TC) meaning that the resistance of the control resistordecreases as temperature increases. In some embodiments where controlresistor R11 has a negative TC, the resistance of the control resistorvaries less than about 10% over an operating temperature range ofcurrent reference 100. The resistance variation of less than about 10%causes a size difference between transistor M2 and flipped gatetransistor M1 to be lower than the size difference used for highervariations in resistance. For example, in some embodiments where controlresistor R11 has a negative TC, a ratio n2/n1 ranges from about 1.0 toabout 8.0, where n2 is a size of transistor M2 and n1 is a size offlipped gate transistor M1.

In some embodiments where control resistor R11 has a negative TC, thecontrol resistor includes p-doped polysilicon. In some embodiments wherecontrol resistor R11 has a negative TC, the control resistor is free ofa silicide layer. A p-doped polysilicon control resistor R11, which isfree of a silicide layer, has a resistance temperature variation of lessthan 5% over the operating temperature range of current reference 100,in some embodiments.

In some embodiments, control resistor R11 has a positive TC meaning thatthe resistance of the control resistor increases as temperatureincreases. In some embodiments where control resistor R11 has a positiveTC, the resistance of the control resistor varies less than about 10%over an operating temperature range of current reference 100. Incomparison with a negative TC, in some embodiments where controlresistor R11 has a positive TC, the size difference between transistorM2 and flipped gate transistor M1 is greater. For example, in someembodiments where control resistor R11 has a positive TC, a ratio n2/n1ranges from about 8.0 to about 40.0.

In some embodiments, control resistor R11 is trimmed at room temperatureduring manufacturing in order to reduce lot-to-lot resistance variationwithin the control resistor. In some embodiments, control resistor R11is trimmed using a passive trimming process. In some embodiments,control resistor R11 is trimmed using an active trimming process. Insome embodiments, control resistor R11 is trimmed using a laser trimmingprocess. The trimming process helps to maintain a high level of controlover manufacturing variations. The trimming process increasesmanufacturing cost and time; however, the trimming process increases aprecision of reference current Iref generated by current reference 100.

FIG. 3 is a schematic diagram of a tracking voltage generator 300 inaccordance with one or more embodiments. Tracking voltage generator 300includes flipped gate transistor M1, transistor M2 and transistor M3similar to tracking voltage generator 100. Tracking voltage generator300 further includes a startup and bias current generator region 310configured to receive an input voltage and to generate a bias current. Afirst current mirror region 320 is configured to generate the firstcurrent I1 for flipped gate transistor M1 based on the bias current fromstartup and bias current generator 310. A second current mirror region330 is configured to receive a mirrored portion of the first current I1and generate the second current I2 for transistor M2. A voltage boxingregion 340 is configured to maintain a voltage drop across transistor M2approximately equal to tracking voltage V_(TRK).

In some embodiments, startup and bias current generator region 310 isomitted. In some embodiments where startup and bias current generatorregion 310 is omitted, tracking voltage generator 300 is configured toreceive the bias current from an external current source.

Startup and bias current generator region 310 is configured to receivean operating voltage VDD. Startup and bias current generator 310 isconnected between the operating voltage VDD and a negative supplyvoltage VSS. Startup and bias current generator region 310 is configuredto generate the bias current Ib along a first line 352 connected tofirst current mirror region 320. First current mirror region 320 isconfigured to receive the operating voltage VDD. A second line 354connected to first current mirror region 320 is connected in series tosecond current mirror 330. A third line 356 connected to first currentmirror 320 is connected in series to flipped gate transistor M1. Afourth line 358 connected to operating voltage VDD through first currentmirror 320 is connected to a first portion of voltage boxing region 340.A fifth line 360 connected to first current mirror 320 is connected inseries with transistor M2. A second portion of voltage boxing region 340is connected to negative supply voltage VSS through second currentmirror region 330. In some embodiments, the operating voltage VDD isgreater than twice the tracking voltage V_(TRK). In some embodiments,negative supply voltage VSS is equal to 0 V. In some embodiments,negative supply voltage VSS is greater or less than 0 V such thatoperating voltage VDD is always referenced to negative supply voltageVSS.

Startup and bias current generator region 310 is configured to generatethe bias current Ib for use by tracking voltage generator 300. Startupand bias current generator region 310 includes a startup resistor R51configured to receive operating voltage VDD. A first bias transistor M52is connected in series with startup resistor R51. A bias resistor R52 isconnected in series to a second bias transistor M51. Bias resistor R52is connected to negative supply voltage VSS. A gate of first biastransistor M52 is connected to a node between second bias transistor M51and bias resistor R52. A gate of second bias transistor M51 is connectedto a node between startup resistor R51 and first bias transistor M52. Asource terminal of first bias transistor M52 is connected to negativesupply voltage VSS. A drain terminal of second bias transistor M51 isconnected in series with first current mirror region 320. In someembodiments, first bias transistor M52 is an NMOS transistor. In someembodiments, second bias transistor M51 is an NMOS transistor. In someembodiments, first bias transistor M52 and second bias transistor M51are in a weak inversion state. A weak inversion state means agate-source voltage Vgs of a transistor is below a threshold voltage ofthe transistor.

Startup resistor R51 is used to provide a direct path from the operatingvoltage VDD to the gate of second bias transistor M51 in order to beginoperation of tracking voltage generator 300. A voltage across biasresistor R52 is at least partially defined based on a gate-sourcevoltage Vgs of first bias transistor M52. The Vgs of first biastransistor M52 is defined at least in part by a voltage utilized toconduct the startup current across startup resistor R51. The startupcurrent of tracking voltage generator 300 is provided by the equationVDD−V(N51)/r51, where VDD is the operating voltage, r51 is acorresponding resistance of startup resistor R51, and V(N51) is given bya sum of a gate-source voltage Vgs of first bias transistor M52 and agate-source voltage Vgs of second bias transistor M51. The bias currentIb is conducted across second bias transistor M51 along the first line352 to current mirror region 320 and is given by the equationV(N52)/r52, where V(N52) is gate-source voltage Vgs of first biastransistor M52 and r52 is a corresponding resistance of bias resistorR52.

First current mirror region 320 is used to provide an integer-ratiomultiple of the bias current Ib to flipped gate transistor M1. Firstcurrent mirror region 320 includes a first mirror transistor M21connected in series with a first mirror resistor R21. First mirrorresistor R21 is connected to the operating voltage VDD. First mirrortransistor M21 is diode-connected. A drain terminal of first mirrortransistor M21 is connected to second bias transistor M51 along thefirst line 352. A second mirror transistor M22 is connected in serieswith a second mirror resistor R22. Second mirror resistor R22 isconnected to the operating voltage VDD. A gate of second mirrortransistor M22 is connected to a gate of first mirror transistor M21. Adrain terminal of second mirror transistor M22 is connected to secondcurrent mirror region 330 along the second line 354. A third mirrortransistor M23 is connected in series with a third mirror resistor R23.Third mirror resistor R23 is connected to the operating voltage VDD. Agate of third mirror transistor is connected to the gate of first mirrortransistor M21. A drain terminal of third mirror transistor M23 isconnected to flipped gate transistor M1 along the third line 356. Afourth mirror transistor M24 is connected in series with a fourth mirrorresistor R24. Fourth mirror resistor R24 is connected to the operatingvoltage VDD. A gate of fourth mirror transistor M24 is connected to thegate of first mirror transistor M21. A drain terminal of fourth mirrortransistor M24 is connected to voltage boxing region 340 along the fifthline 360. The drain terminal of fourth mirror transistor M24 is alsoconnected to transistor M2 along the fifth line 360. In someembodiments, each of first mirror transistor M21, second mirrortransistor M22, third mirror transistor M23 and fourth mirror transistorM24 are PMOS transistors.

First current mirror region 320 is configured to receive the biascurrent Ib from startup and bias current generator region 310 along thefirst line 352 and mirror the bias current Ib along the second line 354,the third line 356 and the fifth line 360. A size of first mirrortransistor M21 is defined as an integer multiple of a first transistorunit size for the first mirror transistor, second mirror transistor M22,third mirror transistor M23 and fourth mirror transistor M24. Secondmirror transistor M22, third mirror transistor M23 and fourth mirrortransistor M24 independently have a size which is an integer multiple ofthe first transistor unit size.

A resistance of first mirror resistor R21 is defined based on the biascurrent Ib conducted across first mirror transistor M21 such that thevoltage drop across the terminals of R21 is greater than 150 mV. Secondmirror resistor R22, third mirror resistor R23 and fourth mirrorresistor R24 independently have a resistance which is based on theinteger-ratio multiples of the first transistor unit size. By using thefirst transistor unit size, a current mirrored across each of the mirrortransistors of first current mirror region is a ratio of the integermultiples of the relative sizes of the transistors multiplied by acurrent Ib across the first mirror transistor. A current I22 acrosssecond mirror transistor M22 is given by (n22/n21)×Ib, where n22 is aninteger multiple of the first transistor unit size for second mirrortransistor M22, n21 is an integer multiple of the first transistor unitsize for first mirror transistor M21, and Ib is the current across thefirst mirror transistor. A current I1 across third mirror transistor M23is given by (n23/n21)>Ib, where n23 is an integer multiple of the firsttransistor unit size for third mirror transistor M23. A current I24across fourth mirror transistor M24 is given by (n24/n21)×Ib, whereinn24 is an integer multiple of the first transistor unit size for fourthmirror transistor M24.

By using the first transistor unit size, a resistance across each of themirror resistors of first current mirror region is a ratio of theinteger multiples of the relative sizes of the transistors multiplied bya resistance r21 corresponding to first mirror resistor R21. Aresistance r22 corresponding to second mirror resistor R22 is given by(n21/n22)×r21, where n22 is an integer multiple of the first transistorunit size for second mirror transistor M22, n21 is an integer multipleof the first transistor unit size for first mirror transistor M21, andr21 is the resistance corresponding to the first mirror resistor. Aresistance r23 corresponding to third mirror resistor R23 is given by(n21/n23)×r21, where n23 is an integer multiple of the first transistorunit size for third mirror transistor M23. A resistance r24corresponding to fourth mirror resistor R24 is given by (n21/n24)×r21,wherein n24 is an integer multiple of the first transistor unit size forfourth mirror transistor M24.

Adjusting sizes of the mirror transistors M21-M24 and the mirrorresistor R21-R24 of first current mirror region 320 enables tuning ofthe current across flipped gate transistor M1, e.g., first current I1(FIG. 1), as well as along the other lines of the first current mirror.For example, third mirror transistor M23 and third mirror resistor R23determine the current across flipped gate transistor M1. In anotherexample, second mirror transistor M22 and second mirror resistor R22determine the current supplied to second mirror region 330. In anadditional example, fourth mirror transistor M24 and fourth mirrorresistor R24 determine the current across transistor M2 and acrosssecond portion of voltage boxing region 340. Tuning of the currentacross flipped gate transistor M1 helps to increase accuracy and controltemperature dependency of tracking voltage V_(TRK) output by trackingvoltage generator 300. The mirror transistors M21-M24 of first currentmirror region 320 are capable of accurately mirroring currents atnano-amp current levels.

Second current mirror region 330 is configured to mirror a current fromfirst current mirror region 320. Second current mirror region 330includes fifth mirror transistor M31 connected in series with fifthmirror resistor R31. Fifth mirror resistor R31 is connected to negativesupply voltage VSS. Fifth mirror transistor M31 is diode-connected. Adrain terminal of fifth mirror transistor M31 is connected to secondmirror transistor M22 along the second line 354. Second current mirrorregion 230 further includes a sixth mirror transistor M32 connected inseries with a sixth mirror resistor R32. Sixth mirror resistor R32 isconnected to negative supply voltage VSS. A gate of sixth mirrortransistor M32 is connected to a gate of fifth mirror transistor M31. Adrain terminal of sixth mirror transistor M32 is connected to voltageboxing region 340 along the fourth line 358. Second current mirrorregion 230 further includes a seventh mirror transistor M33 connected inseries with a seventh mirror resistor R33. Seventh mirror resistor R33is connected to negative supply voltage VSS. A gate of seventh mirrortransistor M33 is connected to a gate of fifth mirror transistor M31 andthe gate of sixth mirror transistor M32. A drain terminal of seventhmirror transistor M33 is connected to transistor M2 and to transistor M3along the fifth line 360. Second current mirror region 230 furtherincludes an eighth mirror transistor M34 connected in series with aneighth mirror resistor R34. Eighth mirror resistor R34 is connected tonegative supply voltage VSS. A gate of eighth mirror transistor M34 isconnected to the gate of fifth mirror transistor M31. A drain terminalof eighth mirror transistor M34 provides bias current I34 to amplifier120 (FIG. 1). In some embodiments, each of fifth mirror transistor M31,sixth mirror transistor M32, seventh mirror transistor M33, and eighthtransistor M34 are NMOS transistors.

Second current mirror region 330 is configured to receive current I22from first current mirror region 320 along the second line 354 andmirror current I22 along the fourth line 358 and along the fifth line360. A size of fifth mirror transistor M31 is defined as an integermultiple of a second transistor unit size. Sixth mirror transistor M32has a size which is an integer multiple of the second transistor unitsize. Seventh mirror transistor M33 also has a size which is an integermultiple of the second transistor unit size. Eighth mirror transistorM34 also has a size which is an integer multiple of the secondtransistor unit size. In some embodiments, the first transistor unitsize is equal to the second transistor unit size. In some embodiments,the first transistor unit size is different from the second transistorunit size.

A resistance of fifth mirror resistor R31 is defined based on thecurrent conducted across fifth mirror transistor M31 such that thevoltage drop across the terminals of R31 is greater than 150 mV. Sixthmirror resistor R32 has a resistance which is based on the integermultiples of the second transistor unit size. Seventh mirror resistorR33 also has a resistance which is based on the integer multiples of thesecond transistor unit size. Eighth mirror resistor R34 also has aresistance which is based on the integer multiples of the secondtransistor unit size.

By using the second transistor unit size, a current mirrored across eachof the mirror transistors of second current mirror region 330 is a ratioof the integer multiples of the relative sizes of the transistorsmultiplied by a current I22 across fifth mirror transistor M31. Acurrent I2 across sixth mirror transistor M32 is given by (n32/n31)×I22,where n32 is an integer multiple of the second transistor unit size forsixth mirror transistor M32, n31 is an integer multiple of the secondtransistor unit size for fifth mirror transistor M31, and I22 is thecurrent across the fifth mirror transistor M31. A current I33 acrossseventh mirror transistor M33 is given by (n33/n31)×I22, where n33 is aninteger multiple of the second transistor unit size for seventh mirrortransistor M33. A current I34 across eighth mirror transistor M34 isgiven by (n34/n31)×I22, where n34 is an integer multiple of the secondtransistor unit size for eighth mirror transistor M34.

By using the second transistor unit size, a resistance across each ofthe mirror resistors of second current mirror region 330 is a ratio ofthe integer multiples of the relative sizes of the transistorsmultiplied by a resistance r31 corresponding to fifth mirror resistorR31. A resistance r32 corresponding to sixth mirror resistor R32 isgiven by (n31/n32)×r31, where n32 is an integer multiple of the secondtransistor unit size for sixth mirror transistor M32, n31 is an integermultiple of the second transistor unit size for fifth mirror transistorM31, and r31 is the resistance corresponding to the fifth mirrorresistor. A resistance r33 corresponding to seventh mirror resistor R33is given by (n31/n33)×r31, where n33 is an integer multiple of thesecond transistor unit size for seventh mirror transistor M33. Aresistance r34 corresponding to eighth mirror resistor R34 is given by(n31/n34)×r31, where n34 is an integer multiple of the second transistorunit size for eighth mirror transistor M34.

Adjusting sizes of the mirror transistors M31-M34 as well as the mirrorresistors R31-R34 of second current mirror region 330 enables tuning ofthe current across transistor M2, e.g., second current I2 (FIG. 1), andtuning of a bias supplied to amplifier 120. For example, sixth mirrortransistor M32 and sixth mirror resistor R32 determine the current I32across a first portion of voltage boxing region 340. In another example,seventh mirror transistor M33 and seventh mirror resistor R33 determinethe current I2 across transistor M2. Tuning of the current acrosstransistor M2 helps to increase accuracy and control temperaturedependency of tracking voltage V_(TRK) output by tracking voltagegenerator 300. In still another example, eighth mirror transistor M34and eighth mirror resistor R34 determine the current I34 configured tobias amplifier 120. The mirror transistors M31-M34 of second currentmirror region 330 are capable of accurately mirroring currents atnano-amp current levels.

Voltage boxing region 340 is configured to maintain a voltage dropacross transistor M2 approximately equal to tracking voltage V_(TRK).Voltage boxing region 340 includes a first boxing transistor M41. Asource terminal of first boxing transistor M41 is connected to sixthmirror transistor M32 along the fourth line 358. A gate of first boxingtransistor M41 is connected to the drain terminal of flipped gatetransistor M1 and is configured to receive current I1. A drain terminalof first boxing transistor M41 is connected to the operating voltageVDD. In some embodiments, first boxing transistor M41 is an NMOStransistor. Voltage boxing region 340 further includes a second boxingtransistor M42. A source terminal of second boxing transistor M42 isconnected to the drain terminal of transistor M2 along the fifth line360. A drain terminal of second boxing transistor M42 is connected tothe negative supply voltage VSS. A gate of second boxing transistor M42is connected to a source terminal of first boxing transistor M41 and isconfigured to receive current I32. In some embodiments, second boxingtransistor M42 is a PMOS transistor.

First boxing transistor M41 is a level-shifting source follower. Firstboxing transistor is biased by current I32 from second current mirrorregion 330. First boxing transistor M41 is configured to performlevel-shifting in a direction of the negative supply voltage VSS. Secondboxing transistor M42 is also a level-shifting source follower. Secondboxing transistor M42 is biased by a difference between a current I24across fourth mirror transistor M24 and current I2 across transistor M2.Current I2 across transistor M2 is less than current I24 across fourthmirror transistor M24. Second boxing transistor M42 is configured toperform level-shifting in a direction of the operating voltage VDD.

First boxing transistor M41 has a size larger than a size of secondboxing transistor M42. A level-shift from the gate of first boxingtransistor M41 to the source terminal of second boxing transistor M42 isa positive value, due to the size difference between the first boxingtransistor and the second boxing transistor as well as the currentdifference between current I32 and the (I24-I2) current across secondboxing transistor M42. The positive value of the level-shifting to thesource terminal of second boxing transistor M42 helps to provide avoltage level at the source terminal of the second boxing transistorsuitable to approximately match a leakage current of transistor M2 to aleakage current of transistor M3. By matching the leakage current oftransistor M2 to the leakage current of M3, tracking voltage V_(TRK)output by tracking voltage generator 300 is maintained at a consistenttemperature variation level. In some embodiments, a voltage level at thesource terminal of second boxing transistor M42 is approximately equalto twice (2 V_(TRK)) the tracking voltage V_(TRK).

In comparison with other boxing regions, voltage boxing region 340 usesnegative level-shifting by first boxing transistor M41 followed bypositive level-shifting by second boxing transistor M42 in order toreduce head-room penalty for tracking voltage generator 300. Head-roompenalty is a difference between the minimum required operating voltageVDD and an output voltage of tracking voltage generator 300. By reducingthe head-room penalty, applicability of tracking voltage generator 300increases over a wider operating voltage VDD range. For example, reducedhead-room penalty increases compatibility of tracking voltage generator300 with lithium-ion batteries or other low voltage power supplies.

FIG. 4 is a schematic diagram of an amplifier 400 in accordance withsome embodiments. Amplifier 400 includes a first amplifier transistorM81 configured to receive tracking voltage V_(TRK). Amplifier 400further includes a second amplifier transistor M82 configured to receivefeedback voltage V_(FB). A third amplifier transistor M83 is connectedin series with first amplifier transistor M81 between operating voltageVDD and negative supply voltage VSS. A fourth amplifier transistor M84is connected in series with second amplifier transistor M82 betweenoperating voltage VDD and negative supply voltage VSS. An output nodeconfigured to output an amplified signal to external circuitry isbetween fourth amplifier transistor M84 and second amplifier transistorM82. In some embodiments, first amplifier transistor M81 and secondamplifier transistor M82 are NMOS transistors. In some embodiments,third amplifier transistor M83 and fourth amplifier transistor M84 arePMOS transistors.

Amplifier 400 is configured to receive the tracking voltage V_(TRK) andthe feedback voltage V_(FB) and to output an amplified signal toexternal circuitry, e.g., control transistor M11 (FIG. 1). In a currentreference, e.g., current reference 100, amplifier 400 is configured tomaintain the tracking voltage V_(TRK) to be substantially equal to thefeedback voltage V_(FB).

First amplifier transistor M81 is configured to receive the trackingvoltage V_(TRK) at a gate. A source of first amplifier transistor M81 isconnected to a current source. In some embodiments, the current sourceis part of a current mirror region of a tracking voltage generator,e.g., eighth mirror transistor M34 (FIG. 3). A drain of first amplifiertransistor M81 is connected to third amplifier transistor M83.

Second amplifier transistor M82 is configured to receive the feedbackvoltage V_(FB) at a gate. A source of second amplifier transistor M82 isconnected to the current source. In some embodiments, the current sourceis part of a current mirror region of a tracking voltage generator,e.g., eighth mirror transistor M34 (FIG. 3). A drain of second amplifiertransistor M82 is connected to the output node and to fourth amplifiertransistor M84.

Third amplifier transistor M83 is a diode-connected transistor. A sourceof third amplifier transistor M83 is connected to operating voltage VDD.A drain of third amplifier transistor M83 is connected to firstamplifier transistor M81. A gate of third amplifier transistor M83 isconnected to a gate of fourth amplifier transistor M84.

A source of fourth amplifier transistor M84 is connected to operatingvoltage VDD. A drain of fourth amplifier transistor M84 is connected tothe output node and to second amplifier transistor M82.

FIG. 5 is a schematic diagram of a current mirror 500 in accordance withsome embodiments. Current mirror 500 is used to provide an integer-ratiomultiple of the reference current Iref to external circuitry. Currentmirror 500 includes a first mirroring transistor M61 connected tooperating voltage VDD. First mirroring transistor M61 isdiode-connected. A drain terminal of first mirroring transistor M61 isconnected to the reference current Iref. A second mirroring transistorM62 is connected to the operating voltage VDD. A gate of secondmirroring transistor M62 is connected to a gate of first mirroringtransistor M61. A drain terminal of second mirroring transistor M62 isconnected to external circuitry. A third mirroring transistor M63 isconnected to the operating voltage VDD. A gate of third mirroringtransistor M63 is connected to the gate of first mirroring transistorM61. A drain terminal of third mirroring transistor M23 is connected toexternal circuitry. A fourth mirroring transistor M64 is connected tothe operating voltage VDD. A gate of fourth mirroring transistor M64 isconnected to the gate of first mirroring transistor M61. A drainterminal of fourth mirroring transistor M64 is connected to externalcircuitry. In some embodiments, each of first mirroring transistor M61,second mirroring transistor M62, third mirroring transistor M63 andfourth mirroring transistor M64 are PMOS transistors. In someembodiments, a number of minoring transistors in current minor 500 isgreater than or less than four.

Current minor 500 is configured to receive the reference current Iref,e.g., from control transistor M11 (FIG. 1). A size of first mirroringtransistor M61 is defined as an integer multiple of a third transistorunit size for the first minoring transistor, second minoring transistorM62, third mirroring transistor M63 and fourth mirroring transistor M64.Second mirroring transistor M62, third minoring transistor M63 andfourth minoring transistor M64 independently have a size which is aninteger multiple of the third transistor unit size. In some embodiments,the third transistor unit size is equal to at least one of the firsttransistor unit size or the second transistor unit size. In someembodiments, the third transistor unit size is different from both thefirst transistor unit size and the second transistor unit size.

By using the third transistor unit size, a current mirrored across eachof the minoring transistors of current minor 500 is a ratio of theinteger multiples of the relative sizes of the transistors multiplied bythe reference current Iref across first mirroring transistor M61. Acurrent I62 across second mirroring transistor M62 is given by(n62/n61)×Iref, where n62 is an integer multiple of the third transistorunit size for second minoring transistor M62, n61 is an integer multipleof the third transistor unit size for first mirroring transistor M61,and Iref is the reference current across the first mirroring transistor.A current I63 across third minoring transistor M63 is given by(n63/n61)×Iref, where n63 is an integer multiple of the third transistorunit size for third minoring transistor M63. A current I64 across fourthminoring transistor M64 is given by (n64/n61)×Iref, wherein n64 is aninteger multiple of the third transistor unit size for fourth mirroringtransistor M64.

Adjusting sizes of the mirroring transistors M61-M64 of current minor500 enables tuning of the current supplied to the external circuitry.The minoring transistors M61-M64 of current mirror 500 are capable ofaccurately mirroring currents at micro-amp current levels.

FIG. 6 is a schematic diagram of a current minor 600 in accordance withsome embodiments. Current mirror 600 is similar to current mirror 500with the addition of minoring resistors R71-R74 between the operatingvoltage VDD and a corresponding mirror transistor M71-M74. In someembodiments, a number of minoring transistors and a number of minoringresistors are greater than or less than four.

Current mirror 600 is configured to receive the reference current Iref,e.g., from control transistor M11 (FIG. 1). A size of first mirroringtransistor M71 is defined as an integer multiple of a fourth transistorunit size for the first mirroring transistor, second mirroringtransistor M72, third mirroring transistor M73 and fourth mirroringtransistor M74. Second mirroring transistor M72, third mirroringtransistor M73 and fourth mirroring transistor M74 independently have asize which is an integer multiple of the fourth transistor unit size. Insome embodiments, the fourth transistor unit size is equal to at leastone of the first transistor unit size or the second transistor unitsize. In some embodiments, the fourth transistor unit size is differentfrom both the first transistor unit size and the second transistor unitsize.

A resistance of first mirroring resistor R71 is defined based on thereference current Iref conducted across first mirroring transistor M71such that the voltage drop across the terminals of R71 is greater than150 mV. By using the fourth transistor unit size, a resistance acrosseach of the mirroring resistors of current mirror 600 is a ratio of theinteger multiples of the relative sizes of the mirroring transistorsmultiplied by a resistance r71 corresponding to first mirroring resistorR71. A resistance r72 corresponding to second mirroring resistor R72 isgiven by (n71/n72)×r71, where n72 is an integer multiple of the fourthtransistor unit size for second mirroring transistor M72, n71 is aninteger multiple of the fourth transistor unit size for first mirroringtransistor M71, and r71 is the resistance corresponding to the firstmirroring resistor R71. A resistance r73 corresponding to thirdmirroring resistor R73 is given by (n71/n73)×r71, where n73 is aninteger multiple of the fourth transistor unit size for third mirroringtransistor M73. A resistance r74 corresponding to fourth mirroringresistor R74 is given by (n71/n74)×r71, wherein n74 is an integermultiple of the fourth transistor unit size for fourth mirroringtransistor M74.

FIG. 7 is a top view of a resistor arrangement 700 in accordance withone or more embodiments. Resistor arrangement 700 has a serpentinestructure. Resistor arrangement 700 includes polysilicon, thin filmsilicon chromium or another suitable resistive material. A minimum widthof the polysilicon in resistor arrangement 700 is defined by a criticaldimension of a formation process. The critical dimension is a smallestdimension which can reliably be formed using the formation process. Insome embodiments, resistor arrangement 700 is formed using a lithographyprocess. By including the serpentine structure and width based on thecritical dimension, resistor arrangement 700 has a higher resistance perunit area in comparison with other approaches which use wider elementsor straight-line layouts. In some embodiments, a resistance of resistorarrangement 700 is on the order of 1 Mega Ohm (MΩ) or greater. In someembodiments, resistor arrangement 700 is used as a resistor unit sizefor resistors in a tracking voltage generator, e.g., tracking voltagegenerator 300 (FIG. 3). For example, if resistance r21 corresponding tofirst mirror resistor R21 is 3 MΩ and the unit resistor size of resistorarrangement 700 is 1 MΩ, the first mirror resistor is formed using threeserial connected resistor arrangements, in some embodiments. The voltagedrop across resistor arrangement 700 is set at a sufficiently high levelto provide current matching in a current mirror, e.g., first currentmirror region 320 or second current mirror region 330 (FIG. 3), and toenable the formation of accurate current mirrors at nanopower levels. Insome embodiments, a voltage drop across resistor arrangement 700 isequal to or greater than 150 millivolts (mV). In some embodiments, atleast one resistor of mirror resistors R21-R24 or R31-R34 is formedhaving resistor arrangement 400. In some embodiments, all mirrorresistors R21-R24 and R31-R34 are formed having resistor arrangement700. Due to the use of nanopower levels, resistances of resistors intracking voltage generator 300 are set as high as possible, in someembodiments.

FIG. 8 is a flowchart of a method 800 of using a current reference inaccordance with some embodiments. Method 800 begins with operation 802in which a tracking voltage is generated. The tracking voltage has atemperature dependence which is substantially equal to a temperaturedependence of a resistor of the current reference. In some embodiments,the tracking voltage is temperature independent. In some embodiments,the tracking voltage increases with an increase in temperature. In someembodiments, the tracking voltage decreases with an increase intemperature. In some embodiments, the tracking voltage is generatedusing a tracking voltage generator, e.g. tracking voltage generator 110(FIG. 1) or tracking voltage generator 300 (FIG. 3).

In operation 804, a conductivity of a control transistor is controlled.The conductivity of the control transistor, e.g., control transistor M11(FIG. 1), is controlled based on the tracking voltage. In someembodiments, the conductivity of the control transistor is controlledusing an amplifier, e.g., amplifier 120 (FIG. 1) or amplifier 400 (FIG.4), connected to a gate of the control transistor. The resistor isconnected in series with the control transistor. A resistance of theresistor has a same temperature dependency as the temperature dependencyof the tracking voltage.

In operation 806, a feedback loop is used to maintain a temperatureindependent reference current. In some embodiments, the feedback loopincludes a feedback voltage applied to an amplifier controlling thecontrol transistor. In some embodiments, the feedback voltage is thevoltage at a node between the resistor and the control transistor. Insome embodiments, the feedback voltage is used to help maintain aconstant ratio between the resistance of the resistor and the trackingvoltage.

In operation 808, the reference current is mirrored to at least oneexternal device. In some embodiments, the reference current is mirroredusing a current mirror, e.g., current mirror 130 (FIG. 1), currentmirror 500 (FIG. 5), or current mirror 600 (FIG. 6). In someembodiments, the reference current is mirrored to a plurality ofexternal devices. In some embodiments, the current mirrored to the atleast one external device is a ratio of the reference current.

One of ordinary skill in the art would recognize that additionaloperations are able to be included in method 800, that operations areable to be omitted, and an order of operations are able to bere-arranged without departing from the scope of this description.

One aspect of this description relates to a current referencecomprising: a control resistor; a tracking voltage generator including:a first, flipped gate transistor having a first size; and a secondtransistor having a second size; wherein the tracking voltage generatoris configured to output a tracking voltage having a first temperaturedependency based on a ratio of the second size to the first size, thetemperature dependency thereby being substantially equal to a secondtemperature dependency of the control resistor; and an amplifier circuitconfigured to receive the tracking voltage and maintain a voltage at afirst terminal of the control resistor, the voltage being substantiallyequal to the tracking voltage; wherein the current reference thereby isconfigured to maintain a reference current through the control resistorat a constant value.

Another aspect of this description relates to current referencecomprising: a control resistor; a tracking voltage generator comprising:a first, flipped gate transistor having a first size; and a secondtransistor having a second size; wherein the tracking voltage generatoris configured to output a tracking voltage having a temperaturedependency based on a ratio of the second size to the first size, thetemperature dependency thereby being substantially equal to atemperature dependency of the control resistor; an amplifier circuitconfigured to receive the tracking voltage and maintain a voltage at afirst terminal of the control resistor, the voltage being substantiallyequal to the tracking voltage; and a current mirror configured toreceive a reference current based on a resistance of the controlresistor and generate a mirrored current from the reference current, thecurrent reference thereby being configured to maintain the mirroredcurrent at a constant value.

Still another aspect of this description relates to a method of methodof using a current reference, the method comprising: receiving atracking voltage with an amplifier; maintaining, using the amplifier, areference current through a control resistor; generating the trackingvoltage from a first, flipped gate transistor having a first size and asecond transistor having a second size, the tracking voltage being basedon a ratio of the second size to the first size, wherein the trackingvoltage has a temperature dependency substantially equal to atemperature dependency of a resistance of the control resistor, thereference current thereby being maintained at a constant value.

It will be readily seen by one of ordinary skill in the art that thedisclosed embodiments fulfill one or more of the advantages set forthabove. After reading the foregoing specification, one of ordinary skillwill be able to affect various changes, substitutions of equivalents andvarious other embodiments as broadly disclosed herein. It is thereforeintended that the protection granted hereon be limited only by thedefinition contained in the appended claims and equivalents thereof.

What is claimed is:
 1. A reference comprising: a control resistor; atracking voltage generator including: a first, flipped gate transistorhaving a first size; and a second transistor having a second size;wherein the tracking voltage generator is configured to output atracking voltage having a first temperature dependency based on a ratioof the second size to the first size, the temperature dependency therebybeing substantially equal to a second temperature dependency of thecontrol resistor; and an amplifier circuit configured to receive thetracking voltage and maintain a voltage at a first terminal of thecontrol resistor, the voltage being substantially equal to the trackingvoltage; wherein the current reference thereby is configured to maintaina reference current through the control resistor at a constant value. 2.The current reference of claim 1, wherein the temperature dependency ofthe tracking voltage is dependent on a relative size of the flipped gatetransistor with respect to the first transistor.
 3. The currentreference of claim 1, wherein a resistance of the control resistor has anegative temperature coefficient.
 4. The current reference of claim 1,wherein a resistance of the control resistor has a positive temperaturecoefficient.
 5. The current reference of claim 1, further comprising acurrent mirror configured to receive the reference current and to mirrorthe reference current to at least one external device.
 6. The currentreference of claim 5, wherein the current mirror comprises: a firstmirroring transistor configured to receive the reference current,wherein the first mirroring transistor is connected to an operatingvoltage; and a second mirroring transistor configured to mirror thereference current to the at least one external device, wherein thesecond mirroring transistor is connected to the operating voltage. 7.The current reference of claim 5, wherein the current mirror comprises:a first mirroring transistor configured to receive the referencecurrent; a first mirroring resistor connected between an operatingvoltage and the first mirroring transistor; a second mirroringtransistor configured to mirror the reference current to the at leastone external device; and a second mirroring resistor connected betweenthe seconding mirroring transistor and the operating voltage.
 8. Thecurrent reference of claim 1, wherein the tracking voltage generatorfurther comprises a boxing region configured to provide a voltage levelat a drain terminal of the first transistor to maintain the firstleakage current substantially equal to the second leakage current. 9.The current reference of claim 1, wherein the amplifier is furtherconfigured to receive a feedback voltage from a node located between thecontrol transistor and the control resistor.
 10. The current referenceof claim 1, wherein the tracking voltage generator further comprises acurrent mirror region configured to provide a bias voltage to theamplifier.
 11. The current reference of claim 1, wherein the trackingvoltage generator further comprises a startup and current generationregion configured to generate a first current to the flipped gatetransistor.
 12. The current reference of claim 11, wherein the trackingvoltage generator further comprises a current mirror region configuredto receive a bias current and provide the first current as aninteger-ratio multiple of the bias current.
 13. A current referencecomprising: a control resistor; a tracking voltage generator comprising:a first, flipped gate transistor having a first size; and a secondtransistor having a second size; wherein the tracking voltage generatoris configured to output a tracking voltage having a temperaturedependency based on a ratio of the second size to the first size, thetemperature dependency thereby being substantially equal to atemperature dependency of the control resistor; an amplifier circuitconfigured to receive the tracking voltage and maintain a voltage at afirst terminal of the control resistor, the voltage being substantiallyequal to the tracking voltage; and a current mirror configured toreceive a reference current based on a resistance of the controlresistor and generate a mirrored current from the reference current, thecurrent reference thereby being configured to maintain the mirroredcurrent at a constant value.
 14. The current reference of claim 13,wherein a resistance of the control resistor has a negative temperaturecoefficient.
 15. The current reference of claim 13, wherein a resistanceof the control resistor has a positive temperature coefficient.
 16. Thecurrent reference of claim 12, wherein the tracking voltage generatorfurther comprises a boxing region configured to provide a voltage levelat a drain terminal of the first transistor to maintain the firstleakage current substantially equal to the second leakage current. 17.The current reference of claim 12, wherein the amplifier is furtherconfigured to receive a feedback voltage from a node located between thecontrol transistor and the control resistor.
 18. A method of operating acurrent reference, the method comprising: receiving a tracking voltagewith an amplifier; maintaining, using the amplifier, a reference currentthrough a control resistor; generating the tracking voltage from afirst, flipped gate transistor having a first size and a secondtransistor having a second size, the tracking voltage being based on aratio of the second size to the first size, wherein the tracking voltagehas a temperature dependency substantially equal to a temperaturedependency of a resistance of the control resistor, the referencecurrent thereby being maintained at a constant value.
 19. The method ofclaim 18, further comprising mirroring the temperature independentreference current to at least one external device.
 20. The method ofclaim 18, wherein the maintaining includes: providing the trackingvoltage to a first input of the amplifier; feeding a feedback-voltageback to a second input of the amplifier, the feedback-voltage being avoltage across the control resistor; and using an output of theamplifier to control a conductivity of a control transistor.